This invention relates to a camera which can perform an exposure control while correcting a displacement of an object light image caused by a shake of a main body of the camera relative to an object.
Recently, there have been proposed various cameras capable of shake correction. In such a camera, an area sensor comprising a multitude of photoelectric conversion elements arrayed in a two-dimensional form such as Charge Coupled Device (CCD) is used to detect a shake amount of an object light image particularly resulting from a camera shake, and a displacement of the object light image is corrected in such a manner as to cancel the shake amount. Also, video cameras and electronic still cameras having a shake correcting function similar to the above are available on the market.
A basic idea of shake correction in the above cameras is to perform shake correction during an image exposure to obtain a photographed image without an image deterioration due to a camera shake. However, it is not always the case that shake correction is completed within the exposure period. For example, when the contrast (spatial frequency) of an instant light image detected by the area sensor is low, an accurate shake amount detection becomes difficult due to the low contrast, thereby disabling the shake correction.
Another cause of obstructing shake correction should be considered in the aspect of a positional relationship between a sensing region of the area sensor and a main object image. FIGS. 14A and 14B show positional relationships between the sensing region shown by the dotted-square and a main object S.
Specifically, in the case where the camera is greatly shaken, the main object S greatly moves in the field of view, and finally goes out of the sensing region, as shown in FIG. 14B. In this case, shake correction cannot be performed. Usually, the sensing region is partly provided in the field of view to ensure quick shake detection. Accordingly, in the case where the main object S is first sensed at an edge of the sensing region, as shown in FIG. 14A, the main object S is liable to go out of the sensing region due to a shake (see FIG. 14B). Consequently, shake correction is disabled.
Also, in a case where a shake correction device is so constructed as to cancel a shake amount by moving a shake correction lens, and detected is a shake amount that may exceed an extreme position where the shake correction lens is difficult to freely move further, shake correction is disabled after the correction lens is moved to the extreme position.
To solve the above problems, there have been proposed the following shake corrections.
U.S. Pat. No. 5,210,563 proposes an arrangement of firing a flash when the contrast of an object image is low. In this arrangement, in the case where the main object S is out of the sensing region as shown in FIG. 14B, shake detection cannot be executed even though the flash is fired. Also, this arrangement is not effective in the case that the main object S is located out of an area of flashlight coverage.
Japanese Unexamined Patent Publication No. HEI 6-308588 discloses an arrangement where, in the case of photographing an object in a low brightness while being hand-held (high likelihood of camera shake), an exposure time smaller than an optimum exposure time is set before starting exposure, and exposure is performed in the shorter time. In this arrangement, the exposure time is determined before starting exposure. The exposure is controlled irrespective of camera shake during exposure. Accordingly, this arrangement cannot finely control the exposure in accordance with a variable camera shake during exposure.
Japanese Unexamined Patent Publication No. HEI 7-261230 discloses an arrangement wherein shake correction is temporarily suspended when a camera shake goes beyond a correctable range of a correction lens, and then resumed upon the correction lens returning to the correctable range. In this arrangement, the exposure is performed for an initially set exposure time even if shake correction is not resumed. Accordingly, this arrangement is not effective in reducing the influence of camera shake to a photographed image.